Although the toxic effect of local anaesthetics on the activity of cerebral cortex has been extensively studied, little is known about their toxic effect on brainstem. Accordingly, the influence of procaine on brainstem auditory evoked potentials (BAEPs) and on the electroencephalogram (EEG) was observed in 15 dogs. After the administration of procaine (15 mg.kg-1, i.v.), the amplitudes of wave I (1.88 +/- 0.56 vs 2.06 +/- 0.61 microV, mean +/- SD) and wave II (1.91 +/- 0.41 vs 2.06 +/- 0.46 microV) in BAEPs increased (P less than 0.05), while the amplitudes of wave III (0.97 +/- 0.27 vs 0.81 +/- 0.24 microV), wave IV (1.15 +/- 0.43 vs 0.85 +/- 0.29 microV) and wave V (1.04 +/- 0.46 vs 0.92 +/- 0.41 microV) were decreased (P less than 0.05), and both the I-III inter-peak latency (1.57 +/- 0.04 vs 1.70 +/- 0.07 msec) and III-V inter-peak latency (2.15 +/- 0.09 vs 2.35 +/- 0.12 msec) were prolonged (P less than 0.01). The EEG changed from an awake pattern to seizure activity. These results suggest that the electrical activity of the brainstem auditory pathway is suppressed even when the cerebral cortex is in an excitatory state at the convulsive stage of procaine toxicity.